This invention relates to pipe and to the method of fabricating such pipe for carrying of abrasive materials under relatively high pressures.
In high pressure flow systems carrying abrasive materials, the inner surface or wall is normally required to have a relatively hard characteristic. However, to provide a high strength pipe, the outer wall should have a ductile characteristic. Such pipe is required in various hydraulic and pneumatic lines for the conveyance of abrasive or semi-solids materials and the like. A dual wall pipe for such applications is particularly disclosed in the U.S. Pat. No. 5,097,585 which issued Mar. 24, 1992 to Robert E. Klemm and is assigned to the assignee of this application. A dual wall pipe particularly adapted for pumping of concrete and other such highly abrasive materials under relatively high pressures and particularly pressure surges is fully disclosed in such patent along with a unique method of forming such pipe. Various other prior art patents are also discussed therein.
As more fully disclosed in the above patent, the dual wall pipe is preferably formed using a special procedure involving induction heating telescoped pipe members which .are located in closed spaced relation and mechanically worked to establish a relatively firm engagement prior to induction heating. The inner pipe is heated to a hardening temperature and rapidly quenched to harden the inner pipe while maintaining a significant degree of ductility in the outer pipe while simultaneously closing the gap between the pipes to produce intimate contact between the two pipes. The result is a pipe unit having a highly hardened inner wall and a ductile outer wall thereby producing a pipe carrying abrasive materials under high pressure as in concrete pumping.
Although such dual wall pipe is widely used and has reached significant acceptance in the trade of concrete pumping and the like, there has been and is presently a demand for a single wall pipe which can replace dual wall pipe with a corresponding cost reduction. With prior art technology, single wall pipes of a very special chemistry and of a wall thickness of at least 3/8 of an inch and generally in a range of 3/8 to 1/2 inch have been available for some period of time. In this technology, the heavy single wall tubing is formed of a special material having a special chemistry of the pipe as the controlling factor. Thus, such pipe requires a very high carbon content with the hardenability controlled by the use of alloys in order to achieve a distinct hardness profile including a hard inner surface with the hardening factor decreasing through the radial wall thickness to the outer surface. The necessity of this profile is well recognized. Thus, if a product had the necessary hardness throughout the entire cross section, the article would be extremely brittle and would be unacceptable for various applications such as concrete pumping systems and the like which includes a high pressure pumping system and particularly high pressure surges. Thus, concrete pumping systems and the like constantly flex and move both as a result of the pressure surges and spikes as well as mechanical and physical contact on the job site.
Although the heavy single wall has found some acceptance, the pipe is reasonably expensive because of the special chemistry involved. It is used primarily where weight restrictions have not been placed on the pipe specifications.
Single wall pipe which has not been hardened is often used for mobile truck mounted concrete pumping system, such as described and shown in the Klemm patent. In such applications, weight of the pipe is of substantial significance and manufacturers of mobile units have made strict weight limitations for the steel pipe for use on their mobile units. Generally, current specification require a thickness no greater than 0.197 inches, and in some specification, no greater than 0.170 inches.
With the special carbon and metallurgical requirements, the commercially available 3/8 inch hardened pipe, for example, claims to have a life of 3 to 8 times that of conventional mild steel. Although a pipe of a lesser thickness is available, the hardness characteristic is only slightly different than that of mild steel and consequently the life is not significantly greater. Another significant difficulty encountered in thin wall pipe is the ability to reproduce the characteristic and thereby provide a reliable pipe unit with necessary cost effective production.
As a result of the current status of the art, there is a continuing demand for a thin wall pipe having the characteristic of dual wall pipes.